The aim of this conference is to provide a forum for authoritative investigators who are actively involved in the various disciplines which define the leading edges of research in hormonal carcinogenesis. It is important for such investigators to continue to meet regularly for the purpose of discussing the latest developments in their individual fields, to analyze the significance of current research, to discuss new tactics for unsolved problems, to critically evaluate current theories, and to develop new theories and approaches as needed. The 1991 conference will focus on the mechanisms which regulate the proliferation, differentiation and transformation of cells. With reference to the specific tissues at risk for hormonally associated neoplasms, the conference will (1) discuss the underlying mechanisms associated with breast and prostate cancer; (2) critically evaluate the role of cell differentiation defects in neoplasia; (3) consider key regulatory mechanisms (both positive and negative) by which hormones regulate growth and differentiation in normal cells, and consider how aberrations in these basic mechanisms might be causally related to neoplasia; (4) examine the regulation of genes involved with steroid biosynthesis, the metabolism of covalent intermediates as a possible mechanism of carcinogenesis; (5) define the neoplastic potential of androgens, estrogens and progestins in terms of the cell biology of development and cancer in target organs. Major themes are: (1) control of cell growth by growth factors and interactions between growth factors and hormones; (2) the role of oncogenes in carcinogenesis; (3) stromal-epithelial interactions during growth control and carcinogenesis, and (4) negative as well as positive growth controls, e.g., inhibitory factors, anti-oncogenes or tumor suppressors. This conference is the major ongoing meeting that discusses hormonal carcinogenesis in a comprehensive, integrated fashion.